1. Technical Field
The present invention relates to an apparatus and method for tracking and displaying a point of attention in a projection image generated from three-dimensional image data, and more specifically to a projection image generation apparatus, method, and program for tracking and displaying, when an area of attention is specified in a projection image, the area of attention in another projection image generated by changing the projection direction.
2. Description of the Related Art
When observing a plurality of medical tomographic images obtained by CT (computed tomography), MRI (nuclear magnetic resonance imaging machine), or the like, it has been widely practiced to generate a projection image by, for example, maximum intensity projection from three-dimensional image data obtained by stacking tomographic images that include an area of attention for observation, because it takes a long time to observe all of the tomographic images one by one and it requires a lot of skill for a doctor to understand a three-dimensional shape of an observation target organ or tissue based only on the tomographic images.
But, front-back information of three-dimensional image data in the projection direction is lost in a maximum intensity projection image, so that a first projection image obtained by performing imaging from a first projection direction alone may not sometimes tell the anteroposterior relationship of organs or tissues in the projection direction represented by the projection image. Consequently, doctors make a comparison between the first projection image and a second projection image (even a third projection image) obtained from a second projection direction (third projection direction) different from the first projection direction to understand a three-dimensional anteroposterior relationship (front-back relationship) of regions of attention.
Japanese Unexamined Patent Publication No. 5 (1993)-189541 proposes a user interface used when generating an image of an object represented as a three-dimensional image projected from a different viewpoint for rotating the object in a sense which is similar to operating the object in a real space (here, the change of projection direction and the rotation of the target object are synonymous). Further, in a case where a projection image is obtained by maximum intensity projection, Japanese Unexamined Patent Publication No. 5 (1993)-277091 proposes a method for spuriously representing a three-dimensional positional relationship, when generating a projection image of blood vessels by subjecting three-dimensional image data obtained by MRA (magnetic resonance angiography) to maximum intensity projection, by detecting an intersection between two or more blood vessels to detect the anteroposterior relationship (front-back relationship) of intersecting blood vessels and decreasing the luminance value of the blood vessel on the back side.
The projection image observation using the conventional methods described above, however, still has a problem that the front-back relationship of organs or tissues of attention in the projection direction in a maximum intensity projection image is difficult to understand. In particular, when observing projection images generated from the second and third projection directions, there has been a problem that where the area of attention in the first projection image is moved in the second or third projection image, or otherwise whether or not the area of attention in the first projection image is displayed in the second or third projection image.
In view of the circumstances described above, it is an object of the present invention to provide an apparatus, method, and program capable of obtaining and displaying, when a point is specified in a first projection image, a position of a first voxel in three-dimensional image data corresponding to the point in a second projection image generated from a projection direction different from that of the first projection image.
It is a further object of the present invention to provide an apparatus, method, and program capable of clearly indicating that, when the first voxel described above and a second voxel in three-dimensional image corresponding to the pixel of the coordinate position in a second projection image to which the first voxel is projected are different, the first and second voxels are different.